Year in Review: The case of the missing craters

July 26, 2016

Editor's note: This story is being highlighted in ASU Now's year in review. To read more top stories from 2016, click here.

When NASA's Dawn spacecraft arrived to orbit the dwarf planet Ceres in March 2015, mission scientists expected to find a heavily cratered body generally resembling the asteroid Vesta, Dawn's previous port of call in the main asteroid belt between Mars and Jupiter.

Instead, as the spacecraft drew near to Ceres, a somewhat different picture began to emerge: Something has happened to Ceres to remove its biggest impact basins.

Now, writing in the online journal Nature Communications, a team of Dawn scientists led by Simone Marchi of the Southwest Research Institute in Boulder, Colorado, reports on their computer simulations of Ceres' history. These suggest that Ceres has experienced significant geological evolution, possibly erasing the large basins. The new finding suggests that Ceres interior structure and geological history are more complex than scientists has previously thought.

The Dawn team includes Arizona State University's David Williams, who is the director of the Ronald Greeley Center for Planetary Studies in ASU's School of Earth and Space Exploration. Wiliams oversees a team of researchers using Dawn data to map the geology of Ceres.

"When we first starting looking at Ceres images, we noticed that there weren't any really large impact basins on the surface," he said. None are larger than 177 miles across. This presents a mystery, he said, because Ceres must have been struck by large asteroids many times over its 4.5-billion-year history.

"Even Vesta, only about half of Ceres' size, has two big basins at its south pole. But at Ceres, all we saw was the Kerwan Basin, just 177 miles in diameter," Williams said. "That was a big red flag that something had happened to Ceres." ASU associate research professor David Williams is on the NASA team of the Dawn spacecraft that is exploring the dwarf planet Ceres, which orbits in the main asteroid belt between Mars and Jupiter. His work helped pinpoint the lack of big craters on dwarf planet Ceres, which led to a new model for Ceres' evolution. Photo by Charlie Leight/ASU Now Download Full Image

The Kerwan Basin's name was proposed by Williams, and it commemorates the Hopi Indian spirit of the sprouting corn.

Wipe out

Dawn lead investigator Marchi noted, "We concluded that a significant population of large craters on Ceres has been obliterated beyond recognition over geological time scales, which is likely the result of Ceres' peculiar composition and internal evolution."

The team's simulations of collisions with Ceres predicted that it should have 10 to 15 craters larger than 250 miles in diameter, and at least 40 craters larger than 60 miles wide. In reality, however, Dawn found that Ceres has only 16 craters larger than 60 miles, and none larger than the 177-mile Kerwan Basin.

Further study of Dawn’s images revealed that Ceres does have three large-scale depressions called "planitiae" that are up to 500 miles wide. These have craters within them that formed in more recent times, but the depressions could be left over from bigger impacts.

One of the depressions, called Vendimia Planitia, is a sprawling area just north of the Kerwan Basin. Vendimia Planitia must have formed much earlier than Kerwan.

Geological activity

So what removed Ceres' large craters and basins?

"If Ceres were highly rocky, we'd expect impact craters of all sizes to be preserved. Remote sensing from Earth, however, told us even before Dawn arrived that the crust of Ceres holds a significant fraction of ice in some form," Williams said.

If Ceres' crust contained a large proportion of ice — especially if mixed with salts — that would weaken the crust and let the topography of a large basin relax and become smoother, perhaps even disappear.

In addition, Williams said, Ceres must have generated some internal heat from the decay of radioactive elements after it formed. This too could also have helped soften or erase large-scale topographic features.

He added, "plus we do see evidence of cryovolcanism — icy volcanism — in the bright spots found scattered over Ceres, especially in Occator Crater." Cryovolcanism behaves like the rocky kind, only at much lower temperatures, where "molten ice" — water or brine — substitutes for molten rock.

"It's possible that there are layers or pockets of briny water in the crust of Ceres," Williams said. "Under the right conditions, these could migrate to the surface and be sources for the bright spots."

For example, in Occator Crater, he pointed out, "the central bright spot is a domed feature which looks as if it has erupted or been pushed up from below."

NASA plans for Dawn to continue orbiting Ceres as the dwarf planet makes its closest approach to the Sun in April 2018. Scientists want to see if the increasing solar warmth triggers any activity or produces detectable changes in Ceres' surface.

"Ceres is revealing only slowly the answers to her many mysteries," Williams said. "Completing the geological maps over the next year, and further analysis of the compositional and gravity data, will help us understand better Ceres' geologic evolution."

The School of Earth and Space Exploration is a unit in ASU's College of Liberal Arts and Sciences

Colors reveal the elevations of craters and depressions on dwarf planet Ceres. The Kerwan Basin is just 177 miles wide, while the depression named Vendimia Planitia spreads about 460 miles across. Scientists were surprised to find Ceres generally lacking in large ancient impact features — the reason may lie in its composition and history.

Photo by NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Occator Crater, seen at upper left, has two bright spots on its floor. These may be places where underground water or brine erupts on the surface as an ice-volcano. If the crust of Ceres contains a lot of ice, that could make it easier for the dwarf planet to erase its oldest and largest impact features.

Robert Burnham

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When NASA's Dawn spacecraft approached the dwarf planet Ceres in March this year, scientists and the public alike were intrigued to see that Ceres has an dark, heavily cratered surface with dozens of bright white spots, large and small. Even more puzzling, the bright spots lie in all kinds of terrain and appear variously as flat patches on the floors of craters and as an isolated peak, in at least...

Deep freeze puts the squeeze on dwarf planet Ceres

ASU scientists have explanation for fresh spots on ancient surface

December 15, 2015

When NASA's Dawn spacecraft approached the dwarf planet Ceres in March this year, scientists and the public alike were intrigued to see that Ceres has an dark, heavily cratered surface with dozens of bright white spots, large and small.

Even more puzzling, the bright spots lie in all kinds of terrain and appear variously as flat patches on the floors of craters and as an isolated peak, in at least one case.

According to Arizona State University's Marc Neveu and Steve Desch, what's emplacing the white material on the surface is what's happening far below. They recently published their model in Geophysical Research Letters to explain what's likely going on.

Ceres orbits the Sun in the main asteroid belt between Mars and Jupiter, at nearly three times Earth's distance from the Sun. Discovered in 1801 and the largest object in the main belt, Ceres is nearly 600 miles wide.

Even before Dawn, scientists knew something was up with Ceres. Earth-based observations and data from orbiting space telescopes had shown it has a surface with water-altered minerals and perhaps a tenuous cloud of water vapor around it.

Dawn's data added to the puzzle. The spots have a brightness that points to ice or salts, and as noted above, they appear as both flat and raised structures in many different types of landscape.

"That was a clue for us," said Neveu of ASU's School of Earth and Space Exploration, where he is a postdoctoral researcher working with Desch, a professor of astrophysics in the school.

"Previously published models suggested that Ceres had subsurface liquid water in the past and possibly until today. We also knew that cryovolcanism on Ceres was definitely an option because a present-day underground ocean would be refreezing and pressurizing liquid water down deep."

Cryovolcanism, or cold volcanism, is partly similar to the ordinary (hot) volcanism that occurs on Earth and other rocky planets. The difference is that in the cryo case, what erupts instead of molten rock is molten ice — liquid water or brine. Water ice would quickly sublimate into the vacuum of space at Ceres' surface, leaving behind salt deposits.

Neveu said the pre-Dawn reports of water vapor were tantalizing clues of surface water or young ice.

"When images released by the Dawn team revealed those bright spots, we immediately thought of a cryovolcanic origin," Neveu said.

The dark surface of dwarf planet Ceres, seen here in an image from NASA's Dawn mission, shows fresh white spots. ASU researchers Marc Neveu and Steve Desch have an explanation how Ceres gets these puzzling features.Download Full Image

Four billion years ago, Ceres formed from a mixture of dust, rocky grains and ices. Today, it has a rocky core under a mantle of ice and rocky fragments. Between the two lie pockets of cold, briny liquid, the source of the white spots on the surface of Ceres. Artwork by Neveu and Desch

Neveu and Desch began with a reasonable assumption that Ceres formed from a mixture of ice and micro-particles of rock and dust. They hypothesized that the rocky particles released heat by radioactive decay. Impacts by meteorites were another likely factor. The heat would melt ice while allowing the denser rock fraction to settle toward the center of Ceres. It would also leave a surface coated with a residue of water-altered minerals.

Said Desch, "Our calculations of Ceres' evolution show that it is just warm enough deep inside Ceres for liquid water to exist." This water, he said, probably has other substances mixed in, making it a brine like seawater. "We calculate a temperature of 240 to 250 Kelvins, or about –27 degrees to –9 degrees Fahrenheit, just warm enough for chloride brines to persist and to be freezing today."

As the brine freezes, Neveu and Desch explained, it will expand (as ice does) and thus raise the pressure on any liquid or briny reservoirs inside Ceres.

"This should drive these fluids up to the surface, where they will erupt as cryovolcanic outflows," according to Neveu.

If cryovolcanism at various places across Ceres’ surface is driven by liquid freezing at depth, this activity may have increased as Ceres has cooled down, making it more likely today.

Cryovolcanism also has a side effect, said Desch: "The eruptions and outflows may contribute to the water vapor being produced at Ceres."